The present invention relates to aluminum alloy extruded materials for structural members of automotive bodies having excellent mechanical strength, impact absorbability, spot weldability, and surface treatment property, and that can be produced at low cost using, as a raw material, recycling aluminum materials, such as recycled aluminum cast scraps of automobiles and aluminum can scraps. The present invention also relates to a method for producing the aluminum alloy extruded materials.
Further, the present invention relates to aluminum alloy extruded materials for structural members of automotive bodies having excellent mechanical strength, bendability, spot weldability, and surface treatment property, and that can be produced at low cost using, as a raw material, recycling aluminum materials, such as recycled aluminum cast scraps of automobiles, recycled aluminum scraps of aluminum cans, and recycled aluminum scraps of aluminum sashes. The present invention also relates to a method for producing the aluminum alloy extruded materials.
Many structural members of automobiles are complicated in shape and are hollow, and since aluminum alloy materials are light in weight and more suitable for extrusion than other materials, use of extruded materials of aluminum alloys as structural members of automotive bodies is now studied. The extruded materials of aluminum alloys are especially suitable since they are not only light but also highly rigid, and then they can absorb energy at the time of a collision through crushing themselves increasing safety.
However, the materials conventionally used in such aluminum alloy extruded materials are mainly 6000-series aluminum alloys, such as 6063 aluminum alloy, and since 6000-series aluminum alloys have relatively low mechanical strength and impact-absorption energy, in comparison with other materials, they have the problem that it is required to increase the thickness of the material shaped. Further, they have the problem that they have poor bendability; that is, when these alloys are subjected to severe bending, cracks occur. Furthermore, there are other problems; for example, the spot weldability is low, requiring a very large electric current for spot welding in the assembling process for automobiles, thereby lowering productivity; and the degreasing property and the chemical conversion property, for example, in the case for surface coating, are poor, thereby making it difficult to secure a coating with good durability. Among structural members of automobiles, particularly those called structural members for bodies, such as side frames, rear frames, center pillars, side sills, and floor frames, are fixed, for example, by spot welding, and they are also exposed to the outside environment, as well as to a corrosive environment, including muddy water. Therefore, the structural members for the bodies are materials that essentially require the chemical conversion susceptivity, since, for example, they are covered by coating for improving the corrosion resistance.
However, hitherto, materials that have various performance properties required for structural members of automotive bodies, such as workability, spot weldability, and surface treatment property, and extrudability and mechanical strength, required for aluminum alloys, and that are also excellent in recycling ability, have not yet been developed.
(i) Although, for example, JP-A-58-31055 (xe2x80x9cJP-Axe2x80x9d means unexamined published Japanese patent application) discloses an aluminum alloy for structure improved in mechanical strength, weldability, and cutting ability/machinability, which comprises 2.3 to 6% by weight of Si, 0.4 to 1.0% by weight of Mg, 0.4 to 1.0% by weight of Mn, small amounts of Zn and Sn, and the balance being made of Al, it is not satisfactory in bendability and spot weldability, and it is greatly different from the present invention, in that it is not one wherein both elements of Cu and Zn are contained, whereby the melting temperature of the aluminum alloy is lowered and the spot weldability and the chemical conversion property (zinc phosphatability (the property of being attached with zinc phosphate)) at the time of pretreatment for coating or the like are improved.
(ii) Further, although JP-A-61-190051 discloses a process for the production of an Al-series hollow extruded material, wherein use is made of an aluminum alloy containing 5 to 15% by weight of Si, and up to 1.0% by weight of Mg, and having an Fe content of 0.5% by weight or less, with Cu, Mn, etc., amounting to 0.25% by weight or less, this aluminum alloy is larger in the amount of added Si than the present invention, and it is an alloy improved in heat resistance and wear resistance properties, such that it is used for high-temperature exposure members of automobiles, rod materials for slide members, and thick extrusion-shape materials, but it is low in spot weldability and surface treatment property, such as zinc phosphatability, and it lacks extrudability. Accordingly, this material is not one that can be used as an extruded material for body structures, as the present invention can.
(iii) Further, JP-A-5-271834 discloses an aluminum alloy fine in crystal grains and stable in artificial aging, which contains 0.2 to 1.2% by weight of Mg, and 1.2 to 2.6% by weight of Si, with the value of {Si (% by weight)xe2x80x94Mg (% by weight)/1.73} being over 0.85 but less than 2.0, and the balance being made of Al. This is an alloy whose composition ratio of Mg to Si is such that Si is in excess in terms of stoichiometric composition, thereby allowing Mg2Si to be formed readily. This is an alloy whose component ranges of Mg and Si in the compositions of conventional JIS 6N01 alloys and AA6005 alloys are simply increased, and the extrudability is excellent, but other properties, i.e. the spot weldability and the surface treatment property, are not satisfactory.
(iv) Furthermore, JP-A-8-225874 describes an aluminum alloy extruded material for automotive structural members that contains 0.5 to 2.5% by weight of Si, 0.2 to 1.0% by weight of Fe, 0.45 to 1.5% by weight of Zn, 0.05 to 1.0% by weight of Cu, and 0.4 to 1.5% by weight of Mn. Although this extruded material is excellent in extrudability, mechanical strength, and surface treatment property, the electrical resistance of the material is low, and the spot weldability is still problematic. That is, in the spot welding in the mass production line of structural members of automotive bodies, the wearing of the welding electrode tip is a problem, and, as the wearing of the electrode tip progresses, the texture of the welded part becomes unstable and the nugget size changes, thereby lowering the strength of the welded part. Therefore, the electrode tip must be replaced frequently, which is a prime cause to adversely affect productivity in the mass production line, and hence the wearing of the welding electrode tip is a prime problem involved in spot welding.
Furthermore, in recent years, in view of environmental problems, effective exploitation of resources, and the like, the importance of recycling of used products is on the increase, leading to activities for legislation to make the recycling of automotive parts obligatory. Hence and the reuse of metal scrap is also being studied in various ways. In particular, there is a need for an established technique for regenerating high-quality materials from recycled aluminum cans, from recycled scraps of aluminum sashes, and from scraps of abandoned automobiles.
Accordingly, an object of the present invention is to provide an aluminum alloy extruded material for structural members of automotive bodies that is excellent in spot weldability and surface treatment property, such as the chemical conversion property and degreasing property, that has high mechanical strength and ductility, and that has excellent impact absorbability.
Further, another object of the present invention provides a method for the production of an aluminum alloy extruded material for structural members of automotive bodies that has excellent spot weldability, surface treatment property, and impact absorbability.
Further, still another object of the present invention provides an extruded material for structural members of automotive bodies that has excellent properties as described above, and that can be produced by using recycled scraps of aluminum cans or recycled scraps of automotive aluminum parts, as a raw material.
Further, another object of the present invention provides an aluminum alloy extruded material for structural members of automotive bodies that has excellent spot weldability and surface treatment property, such as the chemical conversion property and degreasing property, that has high mechanical strength and ductility, and that is excellent in bendability.
Further, still another object of the present invention is to provide a method for the production of such an aluminum alloy extruded material for structural members of automotive bodies that has excellent spot weldability, surface treatment property, and bendability.
Further, another object of the present invention provides an extruded material for structural members of automotive bodies that has excellent properties as described above, and that can be produced by using recycled scraps of aluminum sashes or scraps of automotive aluminum parts, as a raw material.
Other and further objects, features, and advantages of the invention will appear more fully from the following description.
In view of the above objects, the inventors of the present invention, having investigated intensively, have found that the above objects can be attained by providing an extruded material obtained by using an aluminum alloy having a specified composition, subjecting the aluminum alloy to a homogenizing treatment under specified conditions, and then hot rolling it. Based on this finding, the present inventors completed the present invention.
That is, according to the present invention, there are provided:
(1) An aluminum alloy extruded material for structural members of automotive bodies, which is composed of an aluminum alloy (hereinafter referred to as the first aluminum alloy) containing more than 2.6% by weight (hereinafter xe2x80x9c% by weightxe2x80x9d being referred simply to as %) but 4.0% or less of Si, more than 0.3% but 1.5% or less of Mg, more than 0.3% but 1.2% or less of Mn, more than 0.3% but 1.2% or less of Zn, more than 0.2% but 1.2% or less of Cu, and more than 0.1% but 1.5% or less of Fe, and the balance being made of Al and unavoidable impurities, having the conductivity of 48% or less based on the IACS and the melting start temperature of 570xc2x0 C. or less;
(2) An aluminum alloy extruded material for structural members of automotive bodies, which is composed of an aluminum alloy (hereinafter referred to as the second aluminum alloy) containing more than 2.6% by weight but 4.0% by weight or less of Si, more than 0.3% by weight but 1.5% by weight or less of Mg, more than 0.3% by weight but 1.2% by weight or less of Zn, more than 0.3% by weight but 1.2% by weight or less of Cu, and more than 0.1% by weight but 1.5% by weight or less of Fe, and containing at least one selected from among Mn in an amount of more than 0.01% by weight but 0.3% by weight or less, Cr in an amount of more than 0.01% by weight but 0.3% by weight or less, Zr in an amount of more than 0.01% by weight but 0.3% by weight or less, and V in an amount of more than 0.01% by weight but 0.3% by weight or less, and the balance being made of Al and unavoidable impurities, having the conductivity of 50% or less based on the IACS and the melting start temperature of 570xc2x0 C. or less;
(3) The aluminum alloy extruded material for structural members of automotive bodies as stated in the above (1) or (2), wherein said aluminum alloy further contains Sr or Sb in an amount of 50 to 500 ppm;
(4) A method for producing the aluminum alloy extruded material for structural members of automotive bodies stated in the above (1), (2), or (3), wherein after an aluminum alloy ingot is subjected to a homogenizing treatment at a billet temperature of over 520xc2x0 C. but 570xc2x0 C. or less for 1 hour or more, it is subjected to a homogenizing treatment by keeping it at a temperature of over 400xc2x0 C. but 520xc2x0 C. or less for 1 hour or more, and thereafter it is cooled, heated again, and subjected to hot extrusion at a billet temperature of over 330xc2x0 C. but 500xc2x0 C. or less;
(5) The method for producing the aluminum alloy extruded material for structural members of automotive bodies as stated in the above (4), wherein at least a part of the material-sliding-surface of the extrusion die is coated with ceramics;
(6) An aluminum alloy extruded material for structural members of automotive bodies produced by the production method as stated in the above (4) or (5), wherein scraps recycled from aluminum cans containing more than 0.5% but 1.2% or less of Mn and more than 1.2% but 2.0% or less of Mg and scraps of automotive aluminum parts containing more than 2.5% but 14% or less of Si are used for at least a part of the aluminum alloy ingot, with the proviso that the aluminum alloy is the above first aluminum alloy; and
(7) An aluminum alloy extruded material for structural members of automotive bodies produced by the production method as stated in the above (4) or (5), wherein scraps recycled from aluminum sashes containing more than 0.2% by weight but 1.0% by weight or less of Mg and scraps of automotive aluminum parts containing more than 2.5% by weight but 14% by weight or less of Si are used for at least a part of the aluminum alloy ingot, with the proviso that the aluminum alloy is the above second aluminum alloy.
Herein, unless otherwise specified, the aluminum alloy used in the present invention includes both the above first and second aluminum alloys.
The first aluminum alloy used in the present invention contains more than 2.6% but 4.0% or less and preferably 2.6 to 3.5% of Si, more than 0.3% but 1.5% or less and preferably 0.3 to 0.8% of Mg, more than 0.3% but 1.2% or less and preferably 0.3 to 0.8% of Mn, more than 0.3% but 1.2% or less and preferably 0.3 to 0.8% of Zn, more than 0.2% but 1.2% or less and preferably 0.2 to 0.8% of Cu, and more than 0.1% but 1.5% or less and preferably 0.1 to 1.0% or less of Fe.
On the other hand, the second aluminum alloy used in the present invention contains more than 2.6% by weight but 4.0% by weight or less and preferably 2.6 to 3.5% by weight of Si, more than 0.3% by weight but 1.5% by weight or less and preferably 0.3 to 0.8% by weight of Mg, more than 0.3% by weight but 1.2% by weight or less and preferably 0.3 to 0.8% by weight of Zn, more than 0.3% by weight but 1.2% by weight or less and preferably 0.3 to 0.8% by weight of Cu, and more than 0.1% by weight but 1.5% by weight or less and preferably 0.1 to 1.0% by weight of Fe, and it further contains at least one selected from among Mn, Cr, Zr, and V with each content amounting to more than 0.01% by weight but 0.3% by weight or less.
The action of each of elements in the aluminum alloy material of the present invention is described.
Si increases the mechanical strength of the aluminum alloy material, as well as secures the required elongation and acts to increase the impact absorption energy. If its content is less than 2.6%, its action is insufficient, whereas if its content is more than 4.0%, the extrusion becomes difficult. Herein the impact absorption energy means the energy that can be absorbed by the compression, the elongation deformation, or the like, and it is evaluated, in the present invention, by the deformation energy required until it is broken in the tensile test. Preferably this value is 0.035 Nm/mm2 or more, and more preferably 0.04 Nm/mm2 or more.
Further, Mg acts to form an intermetallic compound with the above Si, Mg2Si (precipitate), to improve the strength. If the amount of Mg is too small, its effect is insufficient, whereas if the amount is too large, the extrudability deteriorates.
Zn lowers the melting point of the alloy to improve the spot weldability, as well as increases the surface reactivity, thereby improving the surface treatment property, such as the degreasing property and the chemical conversion property. When Zn is increased in conventional aluminum alloy extruded materials for automotive structural members, a difficulty arises that the self-corrosion-resistance is deteriorated. On the other hand, in the composition of the present invention, since the surface coating is applied, that difficulty is prevented, by widening the allowable range where the self-corrosion resistance is lowered. If the amount of Zn is too small, the spot surface treatment property becomes unsatisfactory and the chemical conversion property is made poor, while if the amount is too large, the corrosion resistance deteriorates.
Cu increases the mechanical strength of the alloy and at the same time lowers the electrical conductivity and the melting point, to improve the spot weldability. Further, it also serves to improve the impact absorption energy by an increase in the strength of the alloy. If the amount of Cu is too small, its action becomes insufficient, while if the amount is too large, the extrusion becomes difficult.
Further, Fe has an action for improving the toughness by refining the crystal grains and an action for increasing the impact absorption energy. If the amount of Fe is too small, its action becomes insufficient, while if the amount is too large, due to the large crystallized phase, the extrudability becomes deteriorated and the impact absorption energy is lowered.
In the first aluminum alloy, Mn increases the mechanical strength, to improve the impact absorption energy. If the amount of Mn is too small, its action becomes insufficient, while if the amount is too large, it forms a large crystallized phase of Alxe2x80x94Mn, thereby lowering the impact absorption energy and the extrudability.
Further, in the second aluminum alloy, Fe in the above proportion, and the elements selected from among Mn, Cr, Zr, and V, have an effect for improving the moldability and the toughness of the alloy by making the crystal grains fine, and as a result improving the bendability.
In the present invention, Sr or Sb may be contained in an amount of 50 to 500 ppm in the aluminum alloy if necessary. This Sr or Sb acts to make the Si grains in the above aluminum alloy fine. If the added amount of Sr or Sb is 50 ppm or less, the refining effect (effect on refining) is insufficient, while if the amount is over 500 ppm, the refining effect is not obtained and it becomes in a so-called overmodification state. Therefore, these elements are added in an amount of 50 to 500 ppm and preferably about 50 to 300 ppm.
Further, to make the Si grains fine, in some cases, Na is used in place of Sr or Sb, but since it causes cracks at the time of hot extrusion, it is not used as far as possible, and use of Sr or Sb is desirable. Although, in view of the refining treatment of Si grains, Na in an amount of about 150 ppm at most is considered sufficient, taking the hot cracking at the time of extrusion into consideration, it is necessary that the amount of its use should be a fraction thereof.
Further, the conductivity of the aluminum alloy extruded material of the present invention is 48% or less based on the IACS and preferably 46% or less based on the IACS in the case wherein the first aluminum alloy is used, and it is 50% or less based on the IACS and preferably 49% or less based on the IACS in the case wherein the second aluminum alloy is used, and the melting start temperature is 570xc2x0 C. or less and preferably 560xc2x0 C. or less. Because of the lower conductivity and the lower melting start temperature, the spot welding in the process for assembling automobile bodies does not require a large electric current and also the electrode tip life can be improved considerably. Therefore, an extruded material for structural members of automotive bodies is made possible that allows spot welding with the welding quality of spot welded parts and the productivity of the welding line secured.
The aluminum alloy extruded material for structural members of automotive bodies of the present invention can be manufactured by subjecting an aluminum alloy ingot having the above composition to a homogenizing treatment under specified conditions, then cooling it, reheating it, and subjecting it to hot extrusion at a prescribed temperature.
The homogenizing treatment at that time can be carried out using any one of (1), (2), or (3): that is, (1) a homogenizing treatment at a temperature of over 450xc2x0 C. but 520xc2x0 C. or less for one hour or more, (2) a homogenizing treatment at a billet temperature of over 520xc2x0 C. but 570xc2x0 C. or less for one hour or more, or (3) a homogenizing treatment at a billet temperature of over 520xc2x0 C. but 570xc2x0 C. or less for one hour or more followed by keeping it at a temperature of over 400xc2x0 C. but 520xc2x0 C. or less for one hour or more.
The homogenizing treatment at a temperature of over 450xc2x0 C. causes Mg2Si to precipitate, which lowers the flow stress. Further if the homogenizing treatment at a high temperature of over 520xc2x0 C. is carried out, the Mn-series precipitation is made coarse, whereby the high-temperature flow stress in the presence of Mg is lessened and the upper limit of the extrusion speed can be elevated.
The homogenizing treatment at a temperature of over 400xc2x0 C. but 520xc2x0 C. or less causes Mg2Si to precipitate, which can further decrease the flow stress, whereby the upper limit of the extrusion speed is further increased.
Further, if the billet heating temperature is too low, the pressure becomes too excessive to carry out the extrusion. If it is too high, the generation of the processing heat at the time of the extrusion causes melting.
The production of the aluminum alloy extruded material for structural members of automotive bodies of the present invention is characterized in that the extrusion speed can be increased more than that of the conventional method. Further, when a part or all of the material sliding surface of the extrusion die is coated with ceramics, the friction resistance is lowered, enabling the upper limit of the speed of the extruded material to be improved by about 20%, which is preferable. More preferably, the ceramics coating is applied to the part having a clearance of at least 3 mm or less, or to all the surface of the die (bearing).
As described above, by subjecting the aluminum alloy ingot having a specified composition to the homogenizing treatment at a specified temperature and an extrusion process, improvement is made with respect to the occurrence of cracks at the time of the extrusion, the excessive extrusion load, and the like, thus that gives increase in the productivity. The cause of the cracks at the time of the extrusion is assumed in such a way that the difference in metal flow causes the speeds at different parts to be different, to result in internal shearing forces in the extruded material and such a tension leads to breakage. In particular, in the case of a hollow member having a center pillar, since a difference in speed is liable to occur from site to site and the generation of processing heat is generally large, the possibility of the generation of cracks is high. On the other hand, according to the method of the present invention, a member having such a shape can be produced at a high extrusion speed without generating cracks.
When the first aluminum alloy is used, although the alloy for use in the present invention is liable to have cracks at the time of hot extrusion thereby leading to a risk of deteriorating the productivity, cracks can be obviated by carrying out the extrusion at a speed determined from the below-shown relationship between the homogenizing treatment and the shape of the extruded material. (V represents the extrusion speed (m/min), and T represents the billet temperature (xc2x0 C.) at the time of the start of the extrusion.)
(1) In the case wherein the homogenizing treatment is carried out at a temperature of more than 450xc2x0 C. but 520xc2x0 C. or less for 1 hour or more:
A hollow member with a center pillar: V less than 14,000/T
A hollow member with no center pillar and a solid member: V less than 20,000/T;
(2) In the case wherein the homogenizing treatment is carried out at a temperature of more than 520xc2x0 C. but 570xc2x0 C. or less for 1 hour or more:
A hollow member with a center pillar: V less than 15,000/T
A hollow member with no center pillar and a solid member: V less than 22,000/T;
(3) In the case wherein the homogenizing treatment is carried out at a temperature of more than 520xc2x0 C. but 570xc2x0 C. or less for 1 hour or more, followed by keeping it at a temperature of more than 400xc2x0 C. but 530xc2x0 C. or less for 1 hour or more:
A hollow member with a center pillar: V less than 16,000/T
A hollow member with no center pillar and a solid member: V less than 24,000/T
As is described above, the extrusion speed is excellent in the order of (3), (2), and (1).
Further, when the second aluminum alloy is used, there is no particular restriction on the speed of the hot extrusion in the present invention, but the below-shown speed given by the relationship between the homogenizing treatment and the shape of the extruded material is particularly preferable. (V represents the extrusion speed (m/min), and T represents the billet temperature (xc2x0 C.) at the time of the start of the extrusion.)
(1) In the case wherein the homogenizing treatment is carried out at a temperature of more than 450xc2x0 C. but 520xc2x0 C. or less for 1 hour or more:
A hollow member with a center pillar: V less than 16,000/T
A hollow member with no center pillar and a solid member: V less than 22,000/T;
(2) In the case wherein the homogenizing treatment is carried out at a temperature of more than 520xc2x0 C. but 570xc2x0 C. or less for 1 hour or more:
A hollow member with a center pillar: V less than 17,000/T
A hollow member with no center pillar and a solid member: V less than 23,000/T;
(3) In the case wherein the homogenizing treatment is carried out at a temperature of more than 520xc2x0 C. but 570xc2x0 C. or less for 1 hour or more, followed by keeping it at a temperature of more than 400xc2x0 C. but 530xc2x0 C. or less for 1 hour or more:
A hollow member with a center pillar: V less than 18,000/T
A hollow member with no center pillar and a solid member: V less than 24,000/T
As is described above, the extrusion speed is excellent in the order of (3), (2), and (1).
In the method for producing an aluminum alloy extruded material for structural members of automotive bodies of the present invention, one of the features is that aluminum cans, aluminum sashes, and aluminum layers of abandoned automobiles can be recycled to use.
Since in the present invention, the first aluminum alloy used contains much Si, Mn, and Zn, and the second aluminum alloy used contains much Si and Zn, various metal scraps can be recycled and utilized as its raw material. Usable recycled scraps include, for example, recycled aluminum cans, aluminum sash scraps, and part scraps including engine scraps of automobiles. Preferably, a recycled material, such as recycled aluminum can scraps containing more than 0.5% but 1.2% or less of Mn and more than 1.2% but 2.0% or less of Mg, recycled aluminum sash scraps containing more than 0.2% but 1.0% or less of Mg, and automotive aluminum-part scraps containing more than 2.5% but 14% or less of Si, are used as part of the raw material. In this case, the recycled material is subjected to a purification treatment if necessary. The purification treatment can be carried out in a usually practiced manner, for example, by the xcex1-phase (xcex1-solid-solution) separating treatment. Such a purification treatment is known per se and is described, for example, in JP-A-7-54061 and JP-A-7-197140, which can be followed.
By using the scraps as described above, the impact absorption energy of the obtained member can be increased. Further, these scraps are relatively easily available and lead to a reduction in cost of the member.
When the first aluminum alloy is used, since the aluminum alloy extruded material for structural members of automotive bodies of the present invention is low in conductivity and melting start temperature, the electrode tip is less worn at the time of spot-welding, and therefore the improvement in the productivity in the assembling process can be attained; further since the degreasing property and the chemical conversion property are good, the surface treatment property is excellent, and in addition since the strength is high and the impact absorption energy is large, such an excellent effect can be exhibited that the thickness can be made decreased. This aluminum alloy extruded material can be used, as a structural member of automotive bodies, in the application where both the spot weldability and the surface treatment property are required, such as a side frame, a rear frame, a center pillar, a side sill, and a floor frame.
Further, when the second aluminum alloy is used, since the aluminum alloy extruded material for structural members of automotive bodies of the present invention is low in conductivity and melting start temperature, the electrode tip is less worn at the time of spot-welding, and therefore the improvement in the productivity in the assembling process can be attained; further since the degreasing property and the chemical conversion property are good, the surface treatment property is excellent, and in addition since the strength is high and the bendability is high, such an excellent effect can be exhibited that cracks are not formed even in high-degree (severe) bending. This aluminum alloy extruded material can be used, as a structural member of automotive bodies, in the application where both the spot weldability and the surface treatment property as well as the bendability are required, such as a side frame, a rear frame, a center pillar, a side sill, and a floor frame.
Further, according to the production method of the present invention, an extruded material without cracks can be produced at a high extrusion speed with good productivity. Further, the aluminum alloy extruded material for structural members of automotive bodies of the present invention can be produced with a high quality at a low cost using recycled aluminum can scraps, recycled aluminum sash scraps, automotive aluminum part scraps, and the like.